The US neighborhood watch volunteer who shot dead an unarmed black teenager is now a free man, but the Justice Department said it is looking into Trayvon Martin's death to determine whether federal prosecutors will file criminal civil rights charges against George Zimmerman.

President Barack Obama and numerous celebrities expressed sorrow at the verdict, which has led to largely peaceful protests across the country by civil rights leaders and others. Hundreds gathered in New York's Times Square and in Los Angeles on Sunday, some chanting "Justice for Trayvon Martin!"

The February 2012 shooting first drew national attention when Zimmerman wasn't arrested for weeks, and the case has continued to raise questions over race and self-defence gun laws.

A jury late Saturday found the 29-year-old Zimmerman not guilty of second-degree murder and declined to convict him on a lesser charge of manslaughter. Zimmerman has said he shot the 17-year-old in self-defense in a nighttime confrontation in his Florida gated community, where Martin was visiting family.

Protests broke out across the US: AP

Jurors were told that Zimmerman was allowed to use deadly force when he shot the teen not only if he actually faced death or bodily harm, but also if he merely thought he did.

With many critics angry over Zimmerman's acquittal, his freedom may be limited. He may also face civil lawsuits from Martin's family.

"He's going to be looking over his shoulder the rest of his life," his brother, Robert Zimmerman Jr. told CNN.

Obama called Martin's death a tragedy for America but asked that everyone respect calls for calm reflection. It was a rare statement from the president on a case that doesn't directly involve the federal government.

"I know this case has elicited strong passions," Obama said. "And in the wake of the verdict, I know those passions may be running even higher. But we are a nation of laws, and a jury has spoken."

More protests were planned Sunday night. Beyonce called at a concert for a moment of silence for Martin. Rapper Young Jeezy released a song in Martin's memory.

Rand Powdrill, 41, said he came to the San Francisco march with about 400 others to "protest the execution of an innocent black teenager."

"If our voices can't be heard, then this is just going to keep going on," he said.

Many, including Martin's parents, said Zimmerman had racially profiled Martin. Zimmerman, whose mother was born in Peru, identifies as Hispanic.

Despite the racially charged nature of the case, race was barely mentioned at the trial.

Martin's family has maintained the teen was not the aggressor, and prosecutors suggested Martin was scared because he was being followed by a stranger. Defence attorneys, however, said Martin knocked Zimmerman down and was slamming the older man's head against the sidewalk when Zimmerman fired his gun.

Prosecutors portrayed Zimmerman as a vigilante who had grown frustrated by break-ins in his neighborhood committed primarily by young black men. Zimmerman assumed Martin was up to no good, prosecutors said.

The Justice Department opened an investigation into Martin's death last year but stepped aside to allow the state prosecution to proceed.

In a statement Sunday, the Justice Department said the criminal section of its civil rights division, the FBI and the local U.S. Attorney's office are continuing to evaluate the evidence.

The department has a long history of using federal civil rights law in an effort to convict defendants who have previously been acquitted in related state cases.

But experience has shown it's almost never easy getting convictions in such high-profile prosecutions.

Alan Vinegrad, a former U.S. Attorney, said federal prosecutors "would have to show not only that the attack was unjustified, but that Mr. Zimmerman attacked Mr. Martin because of his race and because he was using a public facility, the street."

The court did not release the racial and ethnic makeup of the six-person jury, but the panel appeared to reporters to be made up of five white women and a sixth who may be Hispanic. The jurors did not talk to reporters after the verdict.